April 29, 2011
How to tell a completely opposite story with a dual-axis chart (This might make you sick!)
Look at the chart below from Chart of the Day. One question, 5 seconds. How much has the variance changed between the two lines from 2001 and 2011?
I’m going to guess you came up with:
- 2001 – The red line is about 1% lower than the blue line
- 2011 - The red line is about 10% lower than the blue line
- Therefore, the total change is 9%
If that's what you guessed, you’re wrong, completely wrong! Why?
- The axes are not synchronized
- The axis on the left is for the blue line, whereas the axis on the right is for the red line
- How can I be sure? There are gray headers at the top of each axis.
This is incredibly confusing and incredibly misleading!
What’s the real answer?
- 2001 – The red line is at 88%, while the blue line is at 79%. The red line is 9% higher than the blue line, not 1% lower like I thought
- 2011 - The red line and blue line are both at about 114%, not the 10% variance I supposed
- Therefore, the total change is actually 0%, not 9%
This is one of the worst and most misleading dual-axis charts I’ve ever seen. At a glance it looked like blue was slightly ahead of red then the gap got really wide in blue’s favor. However, it’s the complete opposite.
Scary, scary, scary!